THE HOOFED MAMMAIS, 249 
turn our attention to the history of the different orders 
of Placental animals, whose pedigree can often be very 
accurately established in detail. 
We must, as already remarked, consider the order of 
Hoofed animals (Ungulata) as the primary group of the 
Indeciduata, or Tuft-placentals; the two other orders, 
Whales and Toothless animals, developed out of them, as 
two diverging groups, probably only at a later period, by 
adaptation to very different modes of life. But it is also 
possible that the animals poor in teeth (Edentata) may be 
of quite a different origin. 
Hoofed animals are in many respects among the most 
important and the most interesting Mammals. They dis- 
tinctly show that a true understanding of the natural 
relationship of animals can never be revealed to us merely 
by the study of living forms, but in all cases only by an 
equal consideration of their extinct and fossil blood-relations 
and ancestors. If, as is usually done, only the living Hoofed 
animals are taken into consideration, it seems quite natural 
to divide them into three entirely distinct orders, namely: 
(1) Horses, or Single-hoofed animals (Solidungula,or Equina); 
(2) Ruminating animals, or Double-hoofed (Bisulca, or Rumi- 
nantia); and (3) Thick-skinned, or Many-hoofed (Multungula, 
or Pachyderma). But as soon as the extinct Hoofed animals 
of the tertiary period are taken into consideration—of which 
animals we possess very numerous and important remains 
—it is seen that this division, but more especially the 
limitation of the Thick-skinned animals, is completely arti- 
ficial, and that these three groups are merely top branches 
lopped from the pedigree of Hoofed animals, which are most 
closely connected by extinct intermediate forms. The one 
